With the increasing demand of users for high-speed data transmission, the Long Term Evolution (LTE) technology is undoubtedly one of the most competitive wireless transmission technologies. However, with the increasing demand for data transmission, increasing the transmission bandwidth and improving the spectrum utilization will be the key to improve the overall performance of the system. In this context, the use of unlicensed frequency bands receives more and more attentions from operators, and the unlicensed frequency bands are considered to be used as supplementary for the existing LTE licensed frequency bands to improve the quality of service to the user. Hence, the first problem to be solved is how to use the unlicensed frequency bands and to coexist with other systems in the unlicensed frequency bands. At present, the consensus reached by the industry is that an unlicensed frequency band needs to be used with the assistance of the licensed spectrum to provide service for a terminal by carrier aggregation.